My simpliest ANSWER:
The person who wins over his fear of dying, while acting for the protection of a third person or an ideal.

I recently read a book (during my summer vacations) with the title of:
"Black Edelweiss - A memoir of combat and consience by a soldier of the Waffen-SS".

In the end of the book I was thinking what makes a soldier to behave like a hero and how can we determine the true nature of a hero, and I came up with these conclusions.

A hero hasn’t lost his willingness to live (he doesn’t suffer from a death-wish)
A hero is aware of his difficult situation and of the danger/risk of his actions
A hero has the choice to act differently (more cautiously)
A hero doesn’t suffer from mental illness or psychological problems
A hero isn’t intoxicate
A hero hasn’t lost the feeling of fear (up to the moment that he starts the heroic act)
A hero when committing a heroic act is not afraid of dying but at the same time he is not committing suicide.

The purpose of a heroic act is providing assistance/protection to someone (-ing) else than us.

The motives of a heroic act are to fulfill the oaths and commitments of the code of honor and companionship (between the comrades soldiers) and to honour the humanitarian values and national ideals.